TBPA is conventionally prepared by brominating phthalic anhydride (PA) in an oleum solution containing 50% or more of free SO.sub.3, and using iodine as a catalyst. This process suffers from severe drawbacks: oleum is a hazardous material which is difficult to employ on an industrial scale, because it is corrosive and emits harmful vapors, and it reacts vigorously on contact with moisture. Acidic and toxic gases evolve during the reaction, and the reaction mixture may foam. The bromination reaction is promoted by working at higher temperatures and higher SO.sub.3 concentrations, under which condition sulfonation is also promoted, and sulfonated by-products must be removed from the product. Furthermore, iodine is an expensive catalyst, the isolation of the product from the oleum is problematic, and corrosion must be taken into account.
Another process, which suffers from similar drawbacks, brominates PA with bromine and hydrogen peroxide in sulfuric acid, with I.sub.2 catalysis [DE-OLS-2 250 550].
Oxidation of tetrabrominated m- and p-xylene has been carried out under specific conditions [U.S. Pat. No. 3,947,494] to obtain phthalic acids of improved quality but with relatively low yields. No oxidation of tetrabrominated o-xylene has been carried out in the art. The much greater difficulty in oxidizing ortho-xylene relative to the other two isomers is clear from the prior art [A.S. Hay et al., J. Org. Chem. 25, 616 (1960)], which teaches that "o-xylene, unlike the other two isomers, cannot readily be oxidized".